1. Field of the Invention
The present invention relates generally to a programmable Josephson voltage standard device, and more particularly, to a programmable Josephson voltage standard device employing microwave driving of multiple frequencies.
2. Background of the Related Art
In general, a Josephson junction 2 has a structure in which an insulator or a non-superconducting metal material is intervened between two superconductors. If microwaves are radiated to the Josephson junction 2, a flat voltage step appears in current-voltage characteristic, as illustrated in FIG. 1. FIG. 1 is a representative graph showing the relationship of current and voltage output when microwaves are applied to the Josephson junction array.
The voltage step illustrated in FIG. 1 is called a “Shapiro step”. In this case, the step voltage is decided by the frequency and the Josephson relation as expressed in Equation 1.
                              v          =                      nf                          K              j                                      ,                            [                  Equation          ⁢                                          ⁢          1                ]            
where n=an integer, f=microwave frequency, and Kj is 483597.9 GHz/V. Precise voltage can be obtained on both positive (+) and negative (−) sides in a quantum mechanical way depending on the direction of applied current. If there is no current, the voltage becomes accurately 0 V since the junction is in superconducting state. In a superconductor-metal-superconductor junction, a desired voltage can be obtained by changing the junction state to each voltage state by using the above method. Accordingly, it is used as a programmable voltage standard.
FIG. 2 is a circuit diagram of a conventional programmable Josephson voltage standard device. FIG. 3 is a circuit diagram showing an improved version of the programmable Josephson voltage standard device illustrated in FIG. 2.
Referring to FIG. 2, in a programmable Josephson voltage standard device employing a superconductor-metal-superconductor (SNS) Josephson junction (i.e., a non-hysteretic Josephson junction), a desired voltage is obtained by turning on/off current biases of each part of an array, where Josephson junctions are connected in series and microwaves are irradiated.
In this case, each of the Josephson junctions has voltage proportional to the driving frequency based on the Josephson relation as expressed in Equation 1. Furthermore, in the frequently used microwave frequency range of 15 to 20 GHz, a voltage of several tens of μV per junction is generated. Thus, in order to obtain voltage of 1.018 V and 10 V, which is used as the standard in the industry, an element having thousands of junctions connected in series needs to be fabricated.
At this time, if microwaves are applied to all the junctions at once, junctions far from the microwave input does not exhibit a good current-voltage characteristic due to microwave attenuation along the waveguide. Thus, in practical devices, in order to obtain a final output voltage, the devices are driven with microwaves being divided into several paths and junction arrays being divided into several regions, as illustrated in FIG. 3, and the respective arrays are again connected in series. The whole array is divided into a given number of junctions so that each part can be biased with current independently. A method of dividing regions includes a binary method of 2n dividing, and a trinary method of 3n dividing.
Part of the array with the applied current bias becomes the voltage step state on the current-voltage characteristic curve illustrated in FIG. 1. Only the current-biased regions have voltages corresponding to the voltage step value with the positive (+) or negative (−) polarity, depending on the polarity of applied current. Regions to which current is not applied become the superconducting states (i.e., 0 V state).
Consequently, the whole output voltage Voutput will become (positive (+) biased junction number—negative (−) biased junction number)×(voltage at the voltage step). At this time, since the whole element groups are driven by microwaves of a single frequency, the entire junctions have the same voltage step. This voltage becomes a basic unit of the output voltage which can be controlled by turn-on/off of bias current.
In the case where voltage smaller than that of the basic unit is to be changed, voltage can be continuously changed by finely changing the frequency of driven microwaves according to the Josephson relation. However, in this case, it is on the assumption that an element must have a very flat frequency characteristic. On the other hand, Change of frequency is slow compared to on/off of current, depending on the characteristics of a microwave generator, and the intermediate state is very unclear. It makes rapid and fine voltage change challenging.
The voltage standard circuit employing the superconducting Josephson device is a device capable of producing precision voltage in a quantum mechanical way, based on the Josephson relation, by irradiating microwave of accurate frequency to a device including Josephson junctions.
The conventional Josephson voltage standard device in which the Josephson junctions of superconductor-insulator-superconductor (SIS) type are connected in series has been used in voltage calibration of the metrology institutions of each country. The SIS junction-based voltage standard device is suitable for generating a fixed micro voltage, but is slow in speed and requires a complicated process when voltage is changed.
Furthermore, the output voltage Voutput is proportional to the number of junctions to which a current bias is applied. Thus it can be considered to serve as a digital-analog converter (DAC) having quantum accuracy.
If change of voltage smaller than that of each junction is to be required, it can be implemented by finely changing the frequency of applied microwaves. In this case, however, there are disadvantages in that devices must have a very flat frequency characteristic and to change the frequency is slow compared to turn-on/off of the current due to the characteristic of the microwave generator. Accordingly, there are disadvantages in frequency change method that the method is relatively slow and inconvenient compared to the method of changing current bias.